The present invention generally relates to in-line fluid heaters. More particularly, the present invention relates to an in-line fluid heater having an anti-corrosive fluid seal between a pass-through fitting and a heating vessel of the heater.
In-line fluid heaters containing an encapsulated electric immersion heater are used where maintaining the liquid's high purity is critical to the process. Such devices are used in industrial manufacturing processes, such as the manufacturing of semiconductors, solar cells, medical devices, or for such processes such as high quality electroplating, or other uses where it is necessary to heat ultra-pure, strongly acidic or caustic solutions at elevated temperatures.
Existing heaters for these applications maintain the fluid's high purity by encapsulating the metal heating wire with an inert plastic, such as perfluoroalkoxy (PFA) or polytetrafluorethylene (PTFE), in order to avoid metallic ions and other contaminants from leaching into the fluid. In addition, the encapsulation also protects the metal heating wire from damage in applications where strongly acidic or caustic solutions are used. The heating elements are then enclosed in a container or heater vessel. The wetted parts, including the container and the heating element encapsulation are made of materials compatible with the processed fluids since they will be in direct contact with the process fluid.
To permit passage of electrical heater lead wires and other wires, such as a ground wire, other tubes, etc. into the heating vessel, standard practice is to drill a hole into the wall of the heating vessel and tap it so that the resulting port has a Female National Pipe Thread (FNPT) configuration. A fitting with a Male National Pipe Thread (MNPT) is now suitable for attachment with the goal of avoiding fluid leakage through the connector fitting. An example of such a prior art heater vessel is described in U.S. Pat. No. 4,835,365 to Etheridge.
Unfortunately, the threads on the pass-through fitting become wetted surfaces. Threads can entrap particles and cause contamination to the process fluids. Other prior art configurations include an O-ring on the fitting to isolate the fitting threads. While this does eliminate the threads as a wetted part, it introduces another wetted material in the form of the O-ring, which can be damaged over time and shed particles into the process fluid.
Maintaining the process fluid free from contamination and particulates is critical to many manufacturing environments. For example, in semiconductor manufacturing, the industry continues looking for methods to reduce the feature sizes on a semiconductor device, and maintaining the process fluid free from contamination and particulates is critical.
Certain liquid pass-through devices, such as in-line fluid heaters and pumps, will be damaged if powered on without fluids. For example, the heating element in an in-line heater designed for heating liquids will typically burn out if operated in a dry condition as the heating element will not be able to dissipate the heat generated without the presence of liquid, thereby damaging the heating element. In a pump, operating in a dry condition can damage the internal mechanical parts by excessive friction or heat.
Therefore, it is common practice to install a liquid level sensor on the output side of such devices and interlock the sensor to the device's power in a manner in which the device can only be powered on when liquid is present. The liquid sensor is placed on the output side of the device to provide a means to electronically indicate when the liquid has flowed through the device, and therefore is safe to operate and power on. One liquid level sensor type commonly used for this application is a capacitive liquid level sensor. These sensors are desirable as they function outside the fluid path and therefore do not come in direct wetted contact with the process fluid, which is particularly desirable when the purity of the process fluid is important.
As the sensor reacts to the presence of liquid present in the tubing, a bracket is required to hold the sensor in place for proper operation. An unreliable sensor reading can result from improperly installing the sensor too close to the tubing or by changing the distance between the sensor and the tubing after calibration. Unreliable sensor readings can also result if the tubing sags, resulting in a change of distance between the tubing and the sensor. Moving the sensor farther upstream from the device could also result in faulty readings since a user can mistakenly place the sensor in a portion of the liquid path (tubing) that contains liquid in all conditions even though the liquid pass-through device does not contain liquid. This can happen, for example, in a section of piping similar to a P-trap or the like.
Therefore, there is a continuing need for an improved non-contaminating in-line heater for process fluids in such industries. There is also a continuing need for an apparatus that will hold a liquid level sensor in such a manner so as to keep the distance between the sensor and the tubing fixed and not allow a user to move the sensor to a different location of the tubing. The present invention fulfills these needs, and provides other related advantages.